Paper coating pigments

ABSTRACT

A coating composition for paper and other substrates, particularly mechanical papers such as lightweight coated (LWC) paper, comprises an aqueous suspension of a particulate pigment together with a binder, wherein the particulate pigment comprises: (a) a first component which is a precipitated calcium carbonate consisting predominantly of aragonitic or rhombohedral particle shapes or of aragonitic and rhombohedral particle shapes in a weight ratio of between around 40:60 and about 60:40 (e.g. about 50:50) aragonitic:rhombohedral, and a second component which is a processed particulate hydrous kaolin clay having a shape factor greater than or equal to about 25 and a steepness greater than or equal to about 20; or (b) a first component which is a fine particulate calcium carbonate consisting predominantly of particles having a generally spherical particle shape, and a second component which is a processed particulate hydrous kaolin clay having a shape factor greater than or equal to about 45 and a mean equivalent particle diameter (d 50 ) less than about 0.5 μm; or (c) a first component which is a precipitated calcium carbonate consisting predominantly of aragonitic and rhombohedral particle shapes in a weight ratio of between about 40:60 and about 60:40 (e.g. about 50:50) aragonitic:rhombohedral, and a second component which is a processed particulate hydrous kaolin clay having a shape factor less than about 25.

RELATED APPLICATION

This PCT application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/377,270, filed May 3,2002, entitled “PAPER COATING PIGMENTS,” the disclosure of which isincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to paper coating pigments. Moreparticularly, the present invention relates to a paper coatingcomposition comprising a processed (“engineered”) particulate kaolinclay and particulate calcium carbonate, to methods for preparing thecomposition, to the use of the composition in paper coating, and tocoated paper prepared using the composition. In this specification theexpression “paper” embraces paper, board, card, paperboard and the like.

BACKGROUND OF THE INVENTION

Coated paper is used for a large range of products including packaging,art paper, brochures, magazines, catalogues and leaflets. Such coatedpaper is required to give a range of properties, including brightness,opacity and sheet gloss, as well as printing performance.

Paper coating compositions are generally prepared by forming a fluidaqueous suspension of particulate pigment material together with abinder and other optional ingredients. Lightweight coated, or LWC, paperis generally coated to a coating weight of from about 5 g·m⁻² to about13 g·m⁻² on each side, and the total grammage, or weight per unit areaof the coated paper is generally in the range of from about 49 g·m⁻² toabout 65 g·m⁻². The coating may conveniently be applied by means of acoating machine including a short dwell time coating head, which is adevice in which a captive pond of coating composition under a slightlyelevated pressure is held in contact with a moving paper web for a timein the range of from 0.0004 second to 0.01 second, before excess coatingcomposition is removed by means of a trailing blade. However, othertypes of coating apparatus may also be used for preparing lightweightcoated paper. LWC paper is generally used for printing magazines,catalogues and advertising or promotional material. The coated paper isrequired to meet certain standards of surface gloss and smoothness. Forexample, the paper is generally required to have a gloss value of atleast about 32, and up to about 70, TAPPI units, and a Parker Print Surfvalue in the range of from about 0.5 μm to about 1.6 μm.

Ultra lightweight coated, or ULWC, paper (otherwise known as lightlightweight coated, or LLWC, paper) is used for catalogues and foradvertising and promotional material sent through the mail to reducemailing costs. The coating weight is generally in the range of from 5g·m⁻² to 7 g·m⁻² per side. The grammage is generally in the range offrom about 35 g·m⁻² to about 48 g·m⁻².

A very important white inorganic pigment for use in preparing coatingcompositions for the manufacture of LWC and ULWC papers is processedparticulate kaolin clay. Large deposits of kaolin clay exist in Devonand Cornwall, England and in the States of Georgia and South Carolina,United States of America. Important deposits also occur in Brazil,Australia, and in several other countries. Kaolin clay consistspredominantly of the mineral kaolinite, together with small proportionsof various impurities. Kaolinite exists in the form of hydrousaluminosilicate crystals in the shape of thin hexagonal plates, butthese plates tend to adhere together face-to-face to form stacks. Theindividual plates may have mean diameters of 1 μm or less, but kaoliniteparticles in the form of stacks of plates may have an equivalentspherical diameter (esd) of up to 10 μm or more. Generally speaking,kaolin clay particles which have an esd of 2 μm or more are in the formof stacks of kaolinite plates, rather than individual plates.

WO-A-99/51815, the disclosure of which is incorporated herein byreference, describes a paper coating pigment comprising a processedparticulate kaolin clay the particles of which (i) have a particle sizedistribution such that at least 80% by weight of the particles have anesd less than 2 μm and not less than 8% by weight of the particles havean esd less than 0.25 μm and (ii) have a shape factor of at least 45.

It is known to replace part of the processed kaolin clay in a papercoating pigment by particulate calcium carbonate. Particulate calciumcarbonate can be obtained from natural sources or can be manufacturedsynthetically. Manufactured calcium carbonate is generally obtained byprecipitation from aqueous solution. Precipitated calcium carbonate(PCC) is obtained in three different principal crystal forms: thevaterite form, which is thermodynamically unstable, the calcite formwhich is the most stable and is also the most abundant naturalcrystalline form, and the aragonite form which is metastable undernormal ambient conditions of temperature and pressure, but converts tocalcite at elevated temperatures.

The aragonite form typically crystallises as long, thin needles(acicular shape) having a typical length:diameter ratio of about 10:1,but the calcite form exists in several different shapes, of which themost commonly found are: the rhombohedral shape, in which the length anddiameter of the crystals are approximately equal and the crystals may beeither aggregated or unaggregated; and the scalenodedral shape, in whichthe crystals are like double, two-pointed pyramids having a typicallength:diameter ratio of about 4:1, and which are generally aggregated.All these forms of calcium carbonate can be prepared by carbonation ofan aqueous lime-containing medium by suitable variation of the processconditions.

Calcium carbonate can be ground to obtain particulate ground calciumcarbonate (GCC), by methods which are well known in the art. GCCparticles have a generally spherical form.

Blends of kaolin clay and aragonitic PCC for use in paper coating areknown in the art. In the early 1960s, Hagemeyer carried out work onvarious pigment blends including kaolin/aragonite blends (TAPPI, March1960, Vol. 43, No. 3, pages 277-288; and TAPPI, February 1964, Vol. 47,No. 2, pages 75-77). Crawshaw et al, 1982 TAPPI Coating ConferenceProceedings 143-164 (1982) describes the effect of PCC shape on certainproperties of kaolin-PCC paper coating blends. U.S. Pat. No. 5,833,747(Bleakley et al.) also describes various kaolin clay/aragonite blends inwhich the aragonite is made by a particular method in which thePCC-containing suspension is at least partially dewatered and subjectedto comminution by high shear attrition grinding with an attritiongrinding medium. WO-A-00/66509 and WO-A-00/66510 (Lyons et al.) describevarious kaolin clay/PCC blends, in which “blocky” kaolin clay is used,by which is stated to mean a shape factor less than 20. The disclosuresof all these references are incorporated herein by reference.

BRIEF DESCRIPTION OF THE INVENTION

It has now been found that a paper having improved properties isobtained when the paper is coated with a paper coating composition whichincludes a pigment comprising a selected particulate processed hydrouskaolin clay and a selected particulate calcium carbonate. Specifically,it has been found that there are synergistic improvements to the gloss,opacity, brightness and smoothness of the paper, or to at least some ofthose parameters, when compared to papers in which the pigment in thecoating is either one of the individual components of the blend.

In accordance with a first aspect of the present invention, there isprovided a 1.0 coating composition for use in producing a gloss coatingon paper and other substrates, the composition comprising an aqueoussuspension of a particulate pigment together with a binder, wherein theparticulate pigment comprises:

-   -   (a) a first component which is a precipitated calcium carbonate        consisting predominantly of aragonitic or rhombohedral particle        shapes or of aragonitic and rhombohedral particle shapes in a        weight ratio of between about 40:60 and about 60:40 (e.g. about        50:50) aragonitic:rhombohedral, and a second component which is        a processed particulate hydrous kaolin clay having a shape        factor greater than or equal to about 25 and a steepness greater        than or equal to about 20; or    -   (b) a first component which is a fine particulate calcium        carbonate consisting predominantly of particles having a        generally spherical particle shape, and a second component which        is a processed particulate hydrous kaolin clay having a shape        factor greater than or equal to about 45 and a mean equivalent        particle diameter (d₅₀) less than about 0.5 μm; or    -   (c) a first component which is a precipitated calcium carbonate        consisting predominantly of aragonitic and rhombohedral particle        shapes in a weight ratio of between about 40:60 and about 60:40        (e.g. about 50:50) aragonitic:rhombohedral, and a second        component which is a processed particulate hydrous kaolin clay        having a shape factor less than about 25.

The coating composition may optionally include further components, asdiscussed in more detail below.

The first and second components of the particulate pigment are suitablypresent in a weight ratio of at least about 10:90 first:secondcomponents, preferably above about 40:60, e.g. about 50:50. It ispreferred that the weight ratio of the first:second components shouldnot be more than about 80:20, more typically not more than about 75:25or about 60:40.

The invention also relates to: methods for preparing the coatingcomposition of the present invention; to pigment blends for use inpreparing the coating composition; to methods for preparing paper coatedwith the said coating composition; and to paper coated with the saidcoating composition.

In one preferred embodiment, the coated paper of the invention is acoated mechanical paper (or groundwood paper), particularly an LWC.

DETAILED DESCRIPTION OF THE INVENTION

The Particulate Pigment—First Component (Calcium Carbonate)

The calcium carbonate component used in the present invention is readilycommercially available, or can be prepared by methods well known in theart.

Examples of commercially available materials include:

Carbonate A. This comprises predominantly aragonitic crystal shapes. Thetypical particle size distribution is as follows: 96.1% by weight lessthan 2 μm; 22.4% by weight less than 0.25 μm. The GE Brightness is 94-98and the d₅₀ is 0.3-0.5 μm. Such a material is OptiCalGloss™, availablefrom the applicant.

Carbonate B. This comprises predominantly rhombohedral crystal shapes.The typical particle size distribution is as follows: 98.5% by weightless than 2 μm; 6.9% by weight less than 0.25 μm. The GE Brightness is95-98 and the d₅₀ is 0.5-0.7 μm. Such a material is OptiCalPrint™,available from the applicant.

Carbonate C. This is an ultrafine GCC and comprises predominantlygenerally spherical particles. The typical particle size distribution issuch that: 93% by weight of the particles are less than 2 μm. The GEBrightness is 96.9. Such a material is Carbital 95™, available from theapplicant.

Carbonate D. This comprises predominantly aragonitic crystal shapes. Thetypical particle size distribution is as follows: 99% by weight lessthan 2 μm; 96% by weight less than 1 μm; 75% by weight less than 0.5 μm;32% by weight less than 0.25 μm. The ISO powder brightness is 94.3.

Carbonate E. This comprises predominantly rhombohedral crystal andshapes. The typical size distribution is as follows: 98% by weight lessthan 2 μm; 90% weight less 1 μm; 39% by weight less than 0.5 μm; 6% byweight less than 0.5 μm. The ISO powder brightness is 95.5. Such amaterial is Albaglos S™, available from SMI.

Carbonate F. This comprises predominantly aragonitic crystal shapes. Thetypical particle size distribution is as follows: 91% by weight lessthan 2 μm; 72% by weight less than 1 μm; 58% by weight less than 0.5 μm;26% by weight less than 0.25 μm. The ISO powder brightness is 94.3.

Carbonate G. This is a lightly ground (65 kWh/t) version of Carbonate F.It comprises predominantly aragonite crystal shapes. The typicalparticle size distribution is as follows: 96% by weight less than 2 μm;86% by weight less than 1 μm; 69% by weight less than 0.5 μm; 30% byweight less than 0.25 μm. The ISO powder brightness is 92.5.

Carbonate H. This is a fully ground (180-200 kWh/t) version of CarbonateF.

Carbonate I. This is a predominantly rhombohedral crystal shape. Thetypical particle size distribution is as follows: 98% by weight lessthan 2 μm; 89% by weight less than 1 μm; 55% by weight less than 1 μm;18% by weight less than 0.25 μm. The ISO powder brightness is 95.9. Sucha material is Faxe Rhombo (0.5 μm)™, available from Faxe.

Carbonate J. The typical particle size distribution is as follows: 99%by weight less than 2 μm; 96% by weight less than 1 μm; 75% by weightless than the 0.5 μm; 26% by weight less than 0.25 μm. The ISO powderbrightness is 93.8. This can be prepared by sand grinding Carbonate F.

Carbonate K. This is a fine GCC and comprises predominantly generallyspherical particles. The typical particle size distribution is such that90% weight of the particles are less than 2 μm and 65% by weight of theparticles are less than 1 μm. The brightness is 97 (GE) or 95 (ISO) andthe d₅₀ is 0.7 μm. Such a material is Carbital 90™, available from theapplicant.

Carbonate L. This is a fine GCC and comprises predominantly generallyspherical particles. The typical size distribution is such that 97-99%by weight of the particles are less than 2 μm; and 87-90% by weight areless than 1 μm. The brightness is 96 (GE) or 94 (ISO) and the d₅₀ is 0.4μm. Such a material is Carbilux™, available from the applicant.

Carbonate M. This is a ground aragonitic PCC. It comprises predominantlyaragonite crystal shapes. The typical particle size distribution is asfollows: 98% by weight less than 2 μm; 94% by weight less than 1 μm; 75%by weight less than 0.5 μm; 30% by weight less than 0.25 μm. The ISOpowder brightness is 93.7.

The methods for preparing PCC generally comprise precipitation using (i)lime and carbon dioxide, (ii) lime and soda or (iii) the Solvay process.A preferred method for preparing aragonitic or rhombohedral PCC uses thefirst method, and includes the step of carbonating an aqueouslime-containing medium to produce an aqueous suspension of a PCC. Theprocess conditions during the precipitation process required generallyto achieve predominantly a preferred crystal form are well known tothose skilled in the art.

For example, predominantly the aragonitic crystal form is precipitatedwhen the aqueous lime-containing medium is prepared by mixing quicklimewith water at a temperature not exceeding 60 degrees Celsius to give anaqueous suspension containing from 0.5 to 3.0 moles of calcium hydroxideper litre of suspension under conditions such that the temperature ofthe suspension increases by not more than 80 Celsius degrees, andcooling the resultant suspension of slaked lime to a temperature in therange from 30 to 50 degrees Celsius, and when the subsequent carbonationinvolves passing a carbon dioxide containing gas through the cooledsuspension at a rate such that not more than 0.02 moles of carbondioxide are supplied per minute per mole of calcium hydroxide toprecipitate calcium carbonate in the suspension, while the temperaturethereof is maintained within the range from 30 to 50 degrees Celsiusuntil the pH has fallen to a value within the range from 7.0 to 7.5.

The precipitate product in the form of an aqueous suspension preferablyhas a viscosity of not more than 500 mPa·s (as measured by a BrookfieldViscometer using a spindle speed of 100 rpm) and is preferably apumpable and flowable slurry.

The aqueous suspension containing the precipitate product initiallyformed may be treated so as to separate partially or fully the aqueoushost medium from the precipitate product solids, e.g. using conventionalseparation processes. For example, processes such as filtration,sedimentation, centrifugation or evaporation may be used. Filtrationusing a filter press is preferred. The separated aqueous medium (e.g.water) may—optionally with further purification or clarification by oneor more chemical, biochemical or mechanical processes which may be knownper se—be recycled for reuse, e.g. in a paper mill (for example, for usein diluting the paper-making stock or for use as showers for washingmachinery). The separated solids may be assessed for quality control bymeasurements taken on samples and subsequently delivered to a storagetank and thereafter supplied as necessary for use in a user application,e.g. in the present invention. The solids containing suspension may bere-diluted for use at the user plant.

It is not necessary for an aqueous suspension containing a PCC productto be dewatered prior to supply for use in a user application, e.g. foruse in a paper mill.

The aqueous suspension or slurry may be delivered to a storage tank ordirectly to the user plant without substantial dewatering.

The PCC typically has a d₅₀ of less than about 0.8 μm, for example lessthan about 0.7 μm, and suitably at least about 0.2 μm, e.g. betweenabout 0.25 μm and about 0.45 μm.

The calcium carbonate component of the pigment products according to thepresent invention preferably has a particle size distribution such thatat least about 90% by weight of the particles have an esd less than 2μm. As used herein the parameter esd is measured in a well known mannerby sedimentation of the particulate material in a fully dispersedcondition in an aqueous medium using a Sedigraph 5100 machine assupplied by Micromeritics Instruments Corporation, Norcross, Ga., USA(telephone: +1 770 662 3620; web-site: www.micromeritics.com), referredto herein as a “Micromeritics Sedigraph 5100 unit”. Such a machineprovides measurements and a plot of the cumulative percentage by weightof particles having an esd less than given esd values.

The PCC employed in the present invention may, if predominantlyaragonite, have in the fully dispersed state a particle sizedistribution such that the percentage P by weight of particles having asize less than xμm, where x is respectively 2 μm, 1 μm, 0.5 μm and 0.25μm is as follows: x (μm) P (%) 2 At least 90 1 At least 75 0.5 At least60 0.25 Between 15 and 40

e.g. the PCC employed in the present invention may have the particlesize distribution as follows: x (μm) P (%) 2 at least 95 1 at least 820.5 at least 66 0.25 between 23 and 33

Alternatively, the PCC employed in the compositions of the presentinvention may, if predominantly rhombohedral, have in the fullydispersed state a particle size distribution such that the percentage Pby weight of particles having a size less than xμm, where x isrespectively 2 μm, 1 μm, 0.5 μm and 0.25 μm, is as follows: X (μm) P (%)2 at least 93 1 at least 86 0.5 at least 22 0.25 Between 5 and 25

e.g. the PCC employed in the compositions of the present invention mayhave the particle size distribution as follows (x and P as definedabove): X (μm) P (%) 2 at least 97 1 at least 90 0.5 at least 25 0.25between 6 and 19

The median equivalent particle diameter of such a rhombohedral PCC maybe from about 0.4 to about 0.6 μm.

The PCC employed in the compositions of the invention may have a GEpowder brightness of at least 90, e.g. at least 92.

The crystal PCC form achieved in practice is unlikely to be 100% of anyselected form. It is quite usual for one crystal form even whenpredominant to be mixed with other forms. Typically, it might beexpected that over 50% by weight of the particles are of the selectedform, for example over about 60% by weight, more preferably at leastabout 80% by weight. Such mixed forms will generally give suitableproduct properties. The expression “predominantly”, when used inreference to the particle shapes or crystal forms, shall be understoodin this way, so that, for example a PCC which is described as“predominantly aragonitic” may also include up to 50% by weight of oneor more other particle shapes or crystal forms, e.g. rhombohedral.

In the present invention, the aragonite crystal form is generallypreferred.

Where a mixture of aragonitic and rhombohedral crystal shapes isrequired according to the present invention, this may be prepared byconventional mixing techniques.

Fine spherical calcium carbonate (ground calcium carbonate or GCC) isproduced from natural or precipitated calcium carbonate by grindingmethods which are well known in the art. The expression “fine” usedherein refers to products in which at least about 80% by weight of theparticles have an esd less than 2 μm, and therefore encompasses the artterm “ultrafine”.

The Particulate Pigment—Second Component (Processed Kaolin Clay)

As discussed in more detail below, the processed kaolin clay componentused in the present invention is readily commercially available, or canbe prepared by methods well known in the art. The kaolin clay componentemployed in the compositions of the present invention may suitably be akaolin having a high brightness, e.g. a GE powder brightness of at least85, e.g. at least 90.

Shape Factor of the Kaolin Clay

A particulate kaolin clay of high shape factor is considered to be more“platey” than a kaolin product of low shape factor. “Shape factor” asused herein is a measure of an average value (on a weight average basis)of the ratio of mean particle diameter to particle thickness for apopulation of particles of varying size and shape as measured using theelectrical conductivity method and apparatus described inGB-A-2240398/U.S. Pat. No. 5,128,606/EP-A-0528078 and using theequations derived in these patent specifications. “Mean particlediameter” is defined as the diameter of a circle which has the same areaas the largest face of the particle. In the measurement method describedin EP-A-0528078 the electrical conductivity of a fully dispersed aqueoussuspension of the particles under test is caused to flow through anelongated tube. Measurements of the electrical conductivity are takenbetween (a) a pair of electrodes separated from one another along thelongitudinal axis of the tube, and (b) a pair of electrodes separatedfrom one another across the transverse width of the tube, and using thedifference between the two conductivity measurements the shape factor ofthe particulate material under test is determined.

As stated above, the shape factor of the particulate kaolin clays usedin the present invention may be greater than, equal to, or less thanabout 25, or may be greater than or equal to about 45, depending on thenature of the first component of the coating composition. Where theshape factor is above about 25, it may preferably be above about 30,more preferably above about 35. Where the shape factor is below about25, it may preferably be between about 5 and about 20.

Mean Equivalent Particle Diameter of the Kaolin Clay

The mean (average) equivalent particle diameter (d₅₀ value) and otherparticle size properties referred to herein for the particulate kaolinclays are as measured by sedimentation of the particulate material in afully dispersed condition in an aqueous medium using a MicromeriticsSedigraph 5100 unit. The mean equivalent particle size d₅₀ is the valuedetermined in this way of the particle esd at which there are 50% byweight of the particles which have an equivalent spherical diameter lessthan that d₅₀ value.

The value of d₅₀ for the particulate kaolin clays used in the presentinvention may be less than, equal to or greater than about 0.5 μm,depending on the nature of the first component. Where the d₅₀ for theparticulate kaolin clay is greater than or equal to about 0.5 μm, it maysuitably be in the range from about 0.5 μm to about 1.5 μm.

Where the d₅₀ for the particulate kaolin is less than or equal to about0.5 μm, it may suitably be in the range from about 0.1 μm to about 0.5μm.

Where the kaolin clay to be used has a shape factor less than about 25,it is preferred that the clay will have a d₅₀ less than about 0.5 μm,for example in the range about 0.1 μm to about 0.3 μm.

Steepness of the Kaolin Clay

The “steepness” of a particulate kaolin clay refers to a parameter ofthe particle size distribution of the kaolin, defined as d₃₀/d₇₀×100,where d₃₀ is the value of the particle esd at which there are 30% byweight of the particles which have an equivalent spherical diameter lessthan that d₃₀ value and d₇₀ is the value of the particle esd at whichthere are 70% by weight of the particles which have an equivalentspherical diameter less than that d₇₀ value.

The steepness of the particulate kaolin clay used in the presentinvention is less than, equal to or greater than about 20, depending onthe nature of the first component. Where the steepness of theparticulate kaolin clay is greater than about 20, it may preferably bebetween about 25 and about 45, e.g. between about 35 and about 45, andtypically less than about 40.

Preparation of the Kaolin Clay

The particulate kaolin clay used in this invention is a processedmaterial derived from a natural source, namely raw natural kaolin claymineral. The processed kaolin clay may typically contain at least 50% byweight kaolinite. For example, most commercially important processedkaolin clays contain greater than 75% by weight kaolinite and maycontain greater than 90%, in some cases greater than 95% by weight ofkaolinite.

The processed kaolin clay used in the present invention may be preparedfrom the raw natural kaolin clay mineral by one or more other processeswhich are well known to those skilled in the art, for example by knownrefining or beneficiation steps.

For example, the clay mineral may be bleached with a reductive bleachingagent, such as sodium hydrosulfite. If sodium hydrosulfite is used, thebleached clay mineral may optionally be dewatered, and optionally washedand again optionally dewatered, after the sodium hydrosulfite bleachingstep.

The clay mineral may be treated to remove impurities, e.g. byflocculation or magnetic separation techniques well known in the art.

The process for preparing the particulate kaolin clay used in thepresent invention may also include one or more comminution steps, e.g.grinding or milling. Light comminution of a coarse kaolin is used togive suitable delamination thereof.

The comminution may be carried out by use of beads or granules of aplastics, e.g. nylon, grinding or milling aid. The coarse kaolin may berefined to remove impurities and improve physical properties using wellknown procedures. The kaolin clay may be treated by a known particlesize classification procedure, e.g. screening and/or centrifuging, toobtain particles having a desired d₅₀ value or particle sizedistribution.

Examples of Kaolin Clays

A number of particulate kaolin clays are commercially available, whichhave the required particle size and shape factor. Alternatively, theparticulate kaolin clays used in the present invention can easily beprepared from commercially available kaolin clays, in ways well known tothe skilled worker, to arrive at the required particle size and shapefactor.

The following particulate processed hydrous kaolin clays for use in thepresent invention may be mentioned. They are used in the Examples below:

Clay A. This has a shape factor of approximately 25 to 35, a d₅₀ of 0.58μm and a steepness of 27. The typical particle size distribution is asfollows: 83% by weight less than 2 μm; 66% by weight less than 1 μm; 47%by weight less than 0.5 μm; 24% by weight less than 0.25 μm. The GEBrightness is 88.9. Such a clay is marketed by the applicant asAstraplate™.

Clay B. This has a shape factor of approximately 33, a d₅₀ of 0.41 μmand a steepness of 36. The typical particle size distribution is asfollows: 94% by weight less than 2 μm; 82% by weight less than 1 μm; 60%by weight less than 0.5 μm; 30% by weight less than 0.25 μm. The ISOBrightness is 86.8.

Clay C. This has a shape factor of approximately 33, a d₅₀ of 0.62 μmand a steepness of 43. The typical particle size distribution is asfollows: 92% by weight less than 2 μm; 73% by weight less than 1 μm; 38%by weight less than 0.5 μm; 14% by weight less than 0.25 μm. The ISOBrightness is 89.1. Such a clay is marketed by the applicant asSupraprint™.

Clay D. This has a shape factor of approximately 56, a d₅₀ of 0.41 μmand a steepness of 32. The typical particle size distribution is asfollows: 92% by weight less than 2 μm; 78.5% by weight less than 1 μm;59% by weight less than 0.5 μm; 31% by weight less than 0.25 μm. The GEBrightness is 88.2. Such a clay is marketed by the applicant as Contour1500™.

Clay E. This has a shape factor of approximately 58, a d₅₀ of 0.46 μmand a steepness of 36. The typical particle size distribution is asfollows: 92% by weight less than 2 μm; 78% by weight less than 1 μm;55.5% by weight less than 0.5 μm; 24.5% by weight less than 0.25 μm. TheGE Brightness is 88.4.

Clay F. This has a shape factor of approximately 25, a d₅₀ of 0.49 μmand a steepness of 24.4. The typical particle size distribution is asfollows: 82% by weight less than 2 μm; 68% by weight less than 1 μm; 50%by weight less than 0.5 μm; 27% by weight less than 0.25 μm. The GEBrightness is 88.1.

Clay G. This has a shape factor of approximately 25-30, a d₅₀ of 0.44 μmand a steepness of 36. The typical particle size distribution is asfollows: 93% by weight less than 2 μm; 80% by weight less than 1 μm; 56%by weight less than 0.5 μm; 27% by weight less than 0.25 μm. The GEBrightness is 87.0. Such a clay is marketed by the applicant asSupragloss 95™.

Clay H. This has a shape factor of approximately 25-30, a d₅₀ of 0.45 μmand a steepness of 30. The typical particle size distribution is asfollows: 90% by weight less than 2 μm; 76% by weight less than 1 μm; 54%by weight less than 0.5 μm; 30% by weight less than 0.25 μm. The GEBrightness is 87.0.

Clay I. This is a “blocky” (low shape factor) paper coating kaolinpigment. This has a shape factor of approximately 12, a d₅₀ of 0.53 μmand a steepness of 47. The typical particle size distribution is asfollows: 95.6% by weight less than 2 μm; 20.5% by weight less than 0.25μm. The GE Brightness is 89.6. Such a clay is marketed by the applicantas Astra-Plus™.

Clay J. This is a “blocky” (low shape factor) paper coating kaolinpigment. This has a shape factor of approximately 11, a d₅₀ of 0.18 μmand a steepness of 36.9. The typical particle size distribution is asfollows: 99% by weight less than 2 μm; 98% by weight less than 1 μm; 92%by weight less than 0.5 μm; 65% by weight less than 0.25 μm. The GEBrightness is 91.3. Such a clay is marketed by Huber as Hubertex 91™.

Clay K. This is a “blocky” (low shape factor) paper coating kaolinpigment. This has a shape factor of approximately 7.8, a d₅₀ of 0.26 μmand a steepness of 37.3. The typical particle size distribution is asfollows: 100% by weight less than 2 μm; 99% by weight less than 1 μm;89% by weight less than 0.5 μm; 51% by weight less than 1 μm. The GEBrightness is 87.7. Such a clay is marketed by Cadam SA (Brazil) asAmazon 88™.

The Binder

The binder of the composition according to the present invention may beselected from binders which are well known in the art. The binder mayform from 4% to 30%, e.g. 8% to 20%, especially 8% to 15%, by weight ofthe solids content of the composition. The amount employed will dependupon the composition and the type of binder, which may itselfincorporate one or more ingredients.

Examples of suitable binders include:

(a) starch: levels typically range from about 4% by weight to about 20%by weight. The starch may suitably be derived from a natural starch,e.g. natural starch obtained from a known plant source, for example,wheat, maize, potato or tapioca. Where starch is employed as a binderingredient, the starch may suitably be modified by one or more chemicaltreatments known in the art. The starch may, for example, be oxidised toconvert some of its —CH₂OH groups to —COOH groups. In some cases thestarch may have a small proportion of acetyl, —COCH₃, groups.Alternatively, the starch may be chemically treated to render itcationic or amphoteric, i.e. with both cationic and anionic charges. Thestarch may also be converted to a starch ether, or hydroxyalkylatedstarch by replacing some —OH groups with, for example, —O.CH₂.CH₂OHgroups, —O.CH₂.CH₃ groups or —O.CH₂.CH₂.CH₂OH groups. A further class ofchemically treated starches which may be used is that known as thestarch phosphates. Alternatively, the raw starch may be hydrolysed bymeans of a dilute acid or an enzyme to produce a gum of the dextrintype. The amount of the starch binder used in the composition accordingto the present invention is preferably from about 4% to about 25% byweight, based on the dry weight of pigment. The starch binder may beused in conjunction with one or more other binders, for examplesynthetic binders of the latex or polyvinyl acetate or polyvinyl alcoholtype. When the starch binder is used in conjunction with another binder,e.g. a synthetic binder, the amount of the starch binder is preferablyfrom about 2% to about 20% by weight, and the amount of the syntheticbinder from about 2% to about 12% by weight, both based on the weight ofdry pigment. Preferably, at least about 50% by weight of the bindermixture comprises modified or unmodified starch.

(b) latex: levels typically range from about 4% by weight to about 20%by weight. The latex may comprise for example a styrene butadiene rubberlatex, acrylic polymer latex, polyvinyl acetate latex, or styreneacrylic copolymer latex.

(c) other binders: levels typically again range from about 4% by weightto about 20% by weight. Examples of other binders include proteinaceousadhesives such as, for example, casein or soy protein; polyvinylalcohol.

Any of the above binders and binder types may be used alone or inadmixture with each other and/or with other binders, if desired.

Optional Additional Components of the Composition

The coating composition according to the present invention may containone or more optional additional components, if desired. Such additionalcomponents, where present, are suitably selected from known additivesfor paper coating compositions. Examples of known classes of optionaladditive are as follows:

(a) one or more cross linkers: e.g. in levels of up to about 5% byweight; for example glyoxals, melamine formaldehyde resins, ammoniumzirconium carbonates.

(b) one or more water retention aids: e.g. in up to about 2% by weight,for example sodium carboxymethyl cellulose, hydroxyethyl cellulose, PVOH(polyvinyl alcohol), starches, proteins, polyacrylates, gums, alginates,polyacrylamide bentonite and other commercially available products soldfor such applications.

(c) one or more viscosity modifiers and/or thickeners: e.g. in levels upto about 2% by weight; for example acrylic associative thickeners,polyacrylates, emulsion copolymers, dicyanamide, triols, polyoxyethyleneether, urea, sulphated castor oil, polyvinyl pyrrolidone, CMC(carboxymethyl celluloses, for example sodium carboxymethyl cellulose),sodium alginate, xanthan gum, sodium silicate, acrylic acid copolymers,HMC (hydroxymethyl celluloses), HEC (hydroxyethyl celluloses) andothers.

(d) one or more lubricity/calendering aids: e.g. in levels up to about2% by weight, for example calcium stearate, ammonium stearate, zincstearate, wax emulsions, waxes, alkyl ketene dimer, glycols.

(e) one or more dispersants: e.g. in levels up to about 2% by weight,for example polyelectrolytes such as polyacrylates and copolymerscontaining polyacrylate species, especially polyacrylate salts (egsodium and aluminium optionally with a group II metal salt), sodiumhexametaphosphates, non-ionic polyol, polyphosphoric acid, condensedsodium phosphate, non-ionic surfactants, alkanolamine and other reagentscommonly used for this function.

(f) one or more antifoamers/defoamers: e.g. in levels up to about 1% byweight, for example blends of surfactants, tributyl phosphate, fattypolyoxyethylene esters plus fatty alcohols, fatty acid soaps, siliconeemulsions and other silicone containing compositions, waxes andinorganic particulates in mineral oil, blends of emulsified hydrocarbonsand other compounds sold commercially to carry out this function.

(g) one or more dry or wet pick improvement additives: e.g. in levels upto about 2% by weight, for example melamine resin, polyethyleneemulsions, urea formaldehyde, melamine formaldehyde, polyamide, calciumstearate, styrene maleic anhydride and others.

(h) one or more dry or wet rub improvement and/or abrasion resistanceadditives: e.g. in levels up to about 2% by weight, for example glyoxalbased resins, oxidised polyethylenes, melamine resins, ureaformaldehyde, melamine formaldehyde, polyethylene wax, calcium stearateand others.

(i) one or more gloss-ink hold-out additives: e.g. in levels up to about2% by weight, for example oxidised polyethylenes, polyethyleneemulsions, waxes, casein, guar gum, CMC, HMC, calcium stearate, ammoniumstearate, sodium alginate and others.

(j) one or more optical brightening agents (OBA) and/or fluorescentwhitening agents (FWA): e.g. in levels up to about 1% by weight, forexample stilbene derivatives.

(k) one or more dyes: e.g. in levels up to about 0.5% by weight.

(l) one or more biocides/spoilage control agents: e.g. in levels up toabout 1% by weight, for example metaborate, sodium dodecylbenenesulphonate, thiocyanate, organosulphur, sodium benzonate and othercompounds sold commercially for this function e.g. the range of biocidepolymers sold by Nalco.

(m) one or more levelling and evening aids: e.g. in levels up to about2% by weight, for example non-ionic polyol, polyethylene emulsions,fatty acid, esters and alcohol derivatives, alcohol/ethylene oxide,sodium CMC, HEC, alginates, calcium stearate and other compounds soldcommercially for this function.

(n) one or more grease and oil resistance additives: e.g. in levels upto about 2% by weight, e.g. oxidised polyethylenes, latex, SMA (styrenemaleic anhydride), polyamide, waxes, alginate, protein, CMC, HMC.

(o) one or more water resistance additives: e.g. in levels up to about2% by weight, e.g. oxidised polyethylenes, ketone resin, anionic latex,polyurethane, SMA, glyoxal, melamine resin, urea formaldehyde, melamineformaldehyde, polyamide, glyoxals, stearates and other materialscommercially available for this function.

(p) one or more additional pigments: The pigment used in the presentinvention, namely the calcium carbonate and kaolin clay system, may beused as the sole pigment in the paper coating compositions, or it may beused in conjunction with one or more other known pigments, such as forexample, calcined kaolin, titanium dioxide, calcium sulphate, satinwhite, talc and so called ‘plastic pigment’. When a mixture of pigmentsis used the calcium carbonate and kaolin clay system is preferablypresent in the composition in an amount of at least about 80% of thetotal dry weight of the mixed pigments.

Any of the above additives and additive types may be used alone or inadmixture with each other and/or with other additives, if desired.

For all of the above additives, the percentages by weight quoted arebased on the dry weight of pigment (100%) present in the composition.Where the additive is present in a minimum amount the minimum amount maybe about 0.01% by weight based on the dry weight of pigment.

The Coating Composition

The coating composition according to the present invention comprises anaqueous suspension of the defined particulate pigment together with thebinder and optionally one or more further additive components, asdiscussed above.

The coating compositions according to the present invention preferablyconsist essentially of an aqueous suspension of the defined particulatepigment, the binder and optionally one or more further additive selectedfrom the list of additive types given above, with less than about 10% byweight of other ingredients.

The solids content of the paper coating composition according to thepresent invention may be greater than about 60% by weight, preferably atleast about 70%, preferably as high as possible but still giving asuitably fluid composition which may be used in coating (e.g. up toabout 80%).

Preparation of the Composition

According to a second aspect of the present invention, there is provideda method for preparing the coating composition of the invention, whichmethod comprises mixing the particulate pigment and the binder in anaqueous liquid medium to prepare a suspension of the solid componentstherein. The coating composition may suitably be prepared byconventional mixing techniques, as will be well known to one of ordinaryskill in this art.

A pigment mixture may initially be formed by mixing aqueous suspensionsof each of the required pigments to form an aqueous suspensionincorporating the mixture of pigments. Such an aqueous suspension may bea dispersed aqueous suspension and the individual aqueous suspensions ofpigments employed to form the mixture may each incorporate a dispersingagent. The dispersing agents employed to disperse the pigments in theindividual aqueous suspensions mixed together, and the concentrations ofsuch suspensions, may be the same or different.

The paper coating composition may be formed by mixing together anaqueous dispersed suspension containing the pigment components, with thebinder and any other optional additional constituents, in a mannerfamiliar to those skilled in the art.

Pigment Blends

According to a third aspect of the present invention, there is provideda pigment composition for use in preparing the coating composition ofthe invention, the pigment composition comprising a mixture ofparticulate materials consisting of or including:

binder, wherein the particulate pigment comprises:

-   -   (a) a first component which is a precipitated calcium carbonate        consisting predominantly of aragonitic or rbombohedral particle        shapes or of aragonitic and rhombohedral particle shapes in a        weight ratio of between about 40:60 and about 60:40 (e.g. about        50:50) aragonitic:rhombohedral, and a second component which is        a processed particulate hydrous kaolin clay having a shape        factor greater than or equal to about 25 and a steepness greater        than or equal to about 20; or    -   (b) a first component which is a fine particulate calcium        carbonate consisting predominantly of particles having a        generally spherical particle shape, and a second component which        is a processed particulate hydrous kaolin clay having a shape        factor greater than or equal to about 45 and a mean equivalent        particle diameter (d₅₀) less than about 0.5 μm; or    -   (c) a first component which is a precipitated calcium carbonate        consisting predominantly of aragonitic and rhombohedral particle        shapes in a weight ratio of between about 40:60 and about 60:40        (e.g. about 50:50) aragonitic:rhombohedral, and a second        component which is a processed particulate hydrous kaolin clay        having a shape factor less than about 25.

The pigment composition may be provided as a dry particulate mixtureconsisting of or including the components defined above, or as asuspension of the particles in a liquid, suitably aqueous, medium.

Paper Coating Process

According to a further aspect of the present invention, there isprovided a method of use of the coating composition, which comprisesapplying the composition to coat a sheet of paper and calendering thepaper to form a gloss coating thereon. Preferably, the gloss coating isformed on both sides of the paper.

Calendering is a well known process in which paper smoothness and glossis improved and bulk is reduced by passing a coated paper sheet betweencalender nips or rollers one or more times. Usually, elastomer coatedrolls are employed to give pressing of high solids compositions. Anelevated temperature may be applied. One or more (e.g. up to about 12,or sometimes higher) passes through the nips may be applied.

Methods of coating paper and other sheet materials, and apparatus forperforming the methods, are widely published and well known. Such knownmethods and apparatus may conveniently be used for preparing coatedpaper according to the present invention. For example, there is a reviewof such methods published in Pulp and Paper International, May 1994,page 18 et seq. Sheets may be coated on the sheet forming machine, i.e.“on-machine”, or “off-machine” on a coater or coating machine. Use ofhigh solids compositions is desirable in the coating method because itleaves less water to evaporate subsequently. However, as is well knownin the art, the solids level should not be so high that high viscosityand levelling problems are introduced.

The methods of coating according to the present invention are preferablyperformed using apparatus comprising (i) a means of applying the coatingcomposition to the material to be coated, viz an applicator; and (ii) ameans for ensuring that a correct level of coating composition isapplied, viz a metering device. When an excess of coating composition isapplied to the applicator, the metering device is downstream of it.Alternatively, the correct amount of coating composition may be appliedto the applicator by the metering device, e.g. as a film press. At thepoints of coating application and metering, the paper web support rangesfrom a backing roll, e.g. via one or two applicators, to nothing (i.e.:just tension). The time the coating is in contact with the paper beforethe excess is finally removed is the dwell time—and this may be short,long or variable.

The coating is usually added by a coating head at a coating station.According to the quality desired, paper grades are uncoated, singlecoated, double coated and even triple coated. When providing more thanone coat, the initial coat (precoat) may have a cheaper formulation andoptionally less pigment in the coating composition. A coater that isapplying a double coating, i.e. a coating on each side of the paper,will have two or four coating heads, depending on the number of sidescoated by each head. Most coating heads coat only one side at a time,but some roll coaters (e.g. film press, gate roll, size press) coat bothsides in one pass.

Examples of known coaters which may be employed include, withoutlimitation, air knife coaters, blade coaters, rod coaters, bar coaters,multi-head coaters, roll coaters, roll/blade coaters, cast coaters,laboratory coaters, gravure coaters, kiss coaters, liquid applicationsystems, reverse roll coaters, curtain coaters, spray coaters andextrusion coaters.

In all examples of coating compositions described in this specification,water is added to the solids to give a concentration of solids which ispreferably such that, when the composition is coated onto a sheet to adesired target coating weight, the composition has a rheology which issuitable to enable the composition to be coated with a pressure (e.g. ablade pressure) of between 1 and 1.5 bar.

Coated Paper Product

According to a further aspect of the present invention, there isprovided a paper coated with a gloss coating which is the dry residue ofa paper coating composition according to the present invention.

The paper, after coating and calendering, may typically have a totalweight per unit area (grammage) in the range from 30 g·m⁻² to 70 g·m⁻²,especially from 49 g·m⁻² to 65 g·m⁻² or 35 g·m⁻² to 48 g·m⁻². The finalcoating preferably has a weight per unit area (coating weight) in therange from 3 g·m⁻² to 20 g·m⁻², especially from 5 g·m⁻² to 13 g·m⁻².Such a coating may be applied to both sides of the paper. Thus, thecoated paper may be LWC or ULWC paper. The paper gloss is preferablygreater than about 45 TAPPI units and the Parker Print Surf value at apressure of 1 MPa of each paper coating is preferably less than about 1μm.

In general, the advantages of the coating composition of the presentinvention are found at all conventional coating weights. However, insome cases it may be found that different combinations of advantages maybe observed at different coating weights. For example, when theparticulate kaolin clay has a relatively high shape factor,simultaneously with a relatively low mean equivalent particle diameterand relatively high steepness, the advantages are found in some cases tobe more pronounced at higher coating weights.

Test Methods

Gloss

The gloss of a coated paper surface may be measured by means of a testlaid down in TAPPI Standard No 480 ts-65. The intensity of lightreflected at an angle from the surface of the paper is measured andcompared with a standard of known gloss value. The beams of incident andreflected light are both at an angle of 75° to the normal to the papersurface. The results are expressed in TAPPI gloss units. The gloss ofthe coated paper according to the present invention may be greater than50, in some cases greater than 55, TAPPI units.

Smoothness

The Parker Print Surf (“PPS”) test provides a measure of the smoothnessof a paper surface, and comprises measuring the rate at which air underpressure leaks from a sample of the coated paper which is clamped, undera known standard force, between an upper plate which incorporates anoutlet for the compressed air and a lower plate, the upper surface ofwhich is covered with a sheet of either a soft or a hard referencesupporting material according to the nature of the paper under test.From the rate of escape of the air, a root mean cube gap in μm betweenthe paper surface and the reference material is calculated. A smallervalue of this gap represents a higher degree of smoothness of thesurface of the paper under test.

Opacity

Opacity, as used herein, is a measure of percent reflectance of incidentlight off a coated substrate. The standard test method is ISO 2471. Theopacity of a sample of paper can be measured by means of an ElrephoDatacolor 3300 spectro-photometer using a wavelength appropriate toopacity measurement. First, a measurement of the percentage of theincident light reflected is made with a stack of at least ten sheets ofpaper over a black cavity (Rinfinity). The stack of sheets is thenreplaced with a single sheet of paper, and a second measurement of thepercentage reflectance of the single sheet on the black cover is made(R). The percentage opacity is then calculated from the formula:Percentage opacity=100×R/Rinfinity.Brightness

The ISO brightness of the coated paper was measured by means of anElrepho Datacolour 2000™ brightness meter fitted with a No 8 filter (457nm wavelength).

The GE Brightness, as expressed herein, is defined in TAPPI StandardT452 and refers to the percentage reflectance to light of a 457 nmwavelength according to methods well known to those of ordinary skill inthe art.

Print Gloss

The print gloss of a coated paper surface is measured through thefollowing standard TAPPI test. The intensity of light reflected at anangle from the surface of the paper is measured and compared with astandard known print gloss value. The beams of incident and reflectedlight are both at an angle of 20 degrees or 75 degrees to the normal tothe paper surface. The results are expressed in TAPPI print gloss units.

DESCRIPTION OF THE EXAMPLES

Embodiments of the present invention will now be described, withoutlimitation, with reference to the following illustrative Examples.

Example 1

In this example, the properties of compositions according to theinvention in which the particulate pigment comprises an aragoniteprecipitated calcium carbonate and a kaolin clay having a shape factorgreater than or equal to 25 and a steepness between 20 and 35, weremeasured in comparison to compositions including single-componentpigments, compositions including a blocky paper coating kaolin clay andcompositions including having a generally spherical particle shape(Carbonate C) or rhombohedral PCC (Carbonate B).

A range of aqueous coating compositions was prepared at about 54% or 58%solids (see Table 1 for details), the solids portion comprising asfollows:

100 parts total pigment (calcium carbonate/kaolin)

8 pph starch (PG280)

8 pph styrene-butadiene rubber latex (Dow)

Acrylic associative thickener as required

1 pph Nopcote C104 (calcium stearate).

The pigments used were:

100% Clay A (Control)

50:50 Clay A:Carbonate B (“OC-Print”)

50:50 Clay A:Carbonate A (“OC-Gloss”)

50:50 Clay A:Carbonate C (“C-95”)

100% Clay I (Control)

50:50 Clay I:OC-Print

50:50 Clay I:OC-Gloss

50:50 Clay I:C-95

100% OC-Print

100% OC-Gloss

100% C-95

Coatings were then applied to a 34.5 g/m² mechanical base paper. A 7.0g/m² coatweight was targeted using the Heli-coater™ 2000 with athree-inch pond head set at a 50° blade angle. The machine speed was 800m min⁻¹. All the colours were coated at constant solids with Brookfieldviscosity adjusted by adjusting the thickener (on average, a dose of ca.0.05 pph was required). The coating colour viscosities achieved with thedifferent pigments are shown in Table 1 below. A range of coat weightsbetween 5 and 10 gm⁻² were obtained and properties interpolated to 7.0gm⁻².

Calendering conditions were as follows:

Instrument: Beloit Supercalender (chrome plated steel roll/cotton roll)

Calendering Pressure: 250 psi (1.7 MPa)

Nips: 3 nips

Temperature: 60° C. TABLE 1 Coating Colour Viscosities for OffsetHercules 4400 rpm Apparent Brookfield Viscosity Pigments mPa · s @ 100rpm mPa · s % Solids 100 Clay A 1008 50.8 58.2  50 Clay A: 50 OC-Print1044 57.8 58.1  50 Clay A: 50 OC-Gloss 1080 50.8 58.2  50 Clay A: 50C-95 1004 49.1 54.4 100 Clay I 1084 68.3 58.2  50 Clay I: 50 OC-Print1072 58.6 58.1  50 Clay I: 50 OC-Gloss 1072 46.1 58.0  50 Clay I: 50C-95 1052 50.6 58.2 100 OC-Print 1056 41.6 58.3 100 OC-Gloss 1064 32.158.1 100 C-95 1044 38.7 58.1ResultsThe sheet properties of gloss, brightness, opacity and PPS smoothnessare shown in Tables 2 (100% pigments) and 3 (50:50 mixtures). In all theresults, the measured properties interpolated to 7.0 gm⁻² are shownfirst, followed by the arithmetic mean of the 100% components (inbrackets), finally the positive or negative synergy obtained. Note thatpositive values represent synergistic improvements in sheet quality andnegative values a deterioration in sheet quality.Using Clay A, brightness and smoothness show synergistic benefits whenblended with all three calcium carbonate types. At 50% calciumcarbonate, aragonite (OptiCalGloss) is the best choice givingsignificantly improved gloss (+3 TAPPI units) along with good brightnessand opacity gains. Carbital 95 gives no significant improvement in glossand opacity.

Blends with Astra-Plus (Clay I) behave differently to Clay A. There areantisynergies in gloss with all three carbonate types. Aragonite(OCGloss) gives antisynergy also in opacity and smoothness, and no gainin brghtness. With the rhombic PCC and GCC, only small synergies inbrightness and smoothness are observed. TABLE 2 Sheet properties of 100%pigment coatings interpolated to 7.0 gm⁻² coat weight OptiCal OptiCalSheet property Clay A Clay I Gloss Print Carbital 95 75° Gloss 44 55 4440 27 Brightness 65.7 68.0 69.6 69.8 67.5 Opacity 87.0 88.0 88.1 88.086.7 PPS Smoothness 1.37 1.17 1.54 1.55 1.86

TABLE 3 Sheet properties of 50/50 blends, arithmetic means and synergyobtained Clay A/OCGloss Clay A/OCPrint Clay A/C-95 Sheet property 50/5050/50 50/50 75° Gloss   47 (44) +3   42 (42) 0   34 (35) −1 Brightness68.7 (67.7) +1.0 68.6 (67.8) +0.8 67.6 (66.6) +1.0 Opacity 87.9 (87.6)+0.3 87.9 (87.5) −0.4 87.2 (86.9) −0.3 PPS 1.41 (1.46) +0.05 1.36 (1.46)+0.1 1.58 (1.62) +0.04 Smoothness Clay I/OCGloss Clay I/OCPrint ClayI/C-95 50/50 50/50 50/50 75° Gloss   45 (49) −4   46 (47) −1   42 (41)−1 Brightness 68.8 (68.8) 0 69.3 (68.9) +0.4 68.1 (67.8) +0.3 Opacity87.8 (88.0) −0.2 88.3 (88.0) +0.3 87.7 (87.4) +0.3 PPS 1.41 (1.36) −0.051.35 (1.36) +0.01 1.41 (1.52) +0.11 Smoothness

Example 2

In this Example, the properties of compositions according to theinvention, in which particulate pigment comprises an aragonitic orrhombohedral precipitated calcium carbonate and a kaolin clay having ashape factor greater than or equal to about 25 and a steepness greaterthan or equal to 20 (clays B, C, D and E), was measured in comparison tocompositions including single-component pigments and compositionsincluding a fine particulate calcium carbonate having a generallyspherical particle shape (Carbital 95).

The following pigments were tested:

100% Clay D

50:50 Clay D:OptiCalGloss

50:50 Clay D:OptiCalPrint

100% OptiCalPrint

100% Clay E

50:50 Clay E:OptiCalGloss

50:50 Clay E:OptiCalPrint

100% Clay C

50:50 Clay C:OptiCalGloss

50:50 Clay C:OptiCalPrint

100% Clay B

50:50 Clay B:OptiCalGloss

50:50 Clay B:OptiCalPrint

50:50 Clay D:C 95

50:50 Clay E:C 95

50:50 Clay C:C 95

50:50 Clay B:C 95

100% C 95.

A range of aqueous coating compositions was prepared at about 53% or 59%solids (see Table 4 for details), the solids portion comprising asfollows:

100 parts pigment (total)

8 pph styrene-butadiene rubber latex (Dow 950)

8 pph hydroxyethyl starch (Penford Gum 280)

1 pph Nopcote C104 (calcium stearate)

The higher solids content of these compositions offers a useful benefitfor dryer-limited paper mills, as it enables them to increase speed.

The colours were coated at 1000 m min⁻¹ onto Calcdonian mechanical LWCbase using a Helicoater 2000D and short dwell head. The coated sampleswere calendered using 8 nips through the Perkins Supercalender at 65° C.and a pressure of 69 bar. The coating colour viscosities achieved withthe different pigments are shown in Table 4 below. TABLE 4 Coatingcolour rheological properties Ferranti Shirley High shear Brookfield 100rpm Solids viscosity mPa · s Viscosity Pigment/Blend wt % 12800 s⁻¹ mPasClay D 53.2 70 720 Clay D/OptiCalGloss 56.1 72 740 50/50 OptiCalGloss57.6 65 860 Clay D/OptiCalPrint 56.1 70 680 50/50 OptiCalPrint 58.9 70660 Clay E 52.8 74 560 Clay E/OptiCalGloss 55.2 72 730 50/50 ClayE/OptiCalPrint 55.2 76 600 50/50 Clay C 57.6 75 680 Clay C/OptiCalGloss57.8 76 780 50/50 Clay C/OptiCalPrint 58.3 77 700 50/50 Clay B 56.4 691020 Clay B/OptiCalGloss 56.7 75 900 50/50 Clay B/OptiCalPrint 56.9 80800 50/50 Clay D/C95 57.5 87 960 50/50 Clay E/C95 57.0 90 780 50/50 ClayC/C95 57.7 98 780 50/50 Clay B/C95 57.0 72 790 50/50 C95 59.7 83 1040Results

Sheet properties are listed for each pigment or pigment blend in Table5. The results are listed in order of increasing coating weight, 6, 8and 10 gm⁻². For the blends, three numbers are listed for each property.These are firstly the measured property, then (in brackets) thearithmetic mean calculated from the results for the 100% components, andfinally the increase or decrease due to blending. This represents themagnitude of any synergistic of antisynergistic effect. If thesynergistic effect results in an improvement, then the result is listedas positive. If the result is a decrease in sheet quality, the result islisted as negative. TABLE 5 Sheet properties at 6, 8 and 10 gsm PigmentGloss B'ness Opacity PPS1000 kPa Clay D 56 69.3 86.7 1.03 62 70.0 87.50.97 67 70.5 88.1 0.91 Clay D/ 52 (50) +2 72.4 (71.3) +1.1 87.5 (87.0)+0.5 1.13 (1.19) +0.06 OptiCalGloss 58 (56) +2 73.6 (72.6) +1.0 88.6(88.1) +0.5 1.02 (1.09) +0.07 50/50 62 (61) +1 74.4 (73.4) +1.0 89.4(88.8) +0.6 0.93 (1.00) +0.07 OptiCalGloss 45 73.3 87.3 1.35 51 75.188.6 1.20 55 76.2 89.5 1.08 Clay D/ 47 (50) −3 72.6 (72.0) +0.4 87.7(87.4) +0.3 1.11 (1.16) +0.05 OptiCalPrint 54 (56) −2 73.9 (73.1) +0.888.7 (88.2) +0.5 1.01 (1.06) +0.05 50/50 60 (60) 0 74.9 (74.0) +0.9 89.4(88.9) +0.5 0.93 (0.99) +0.06 OptiCalPrint 44 74.7 88.0 1.28 49 76.288.9 1.16 53 77.4 89.6 1.07 Clay D/ 44 (44) 0 71.8 (70.7) +1.1 87.0(86.5) +0.5 1.10 (1.28) +0.18 C95 50 (50) 0 72.7 (71.6) +1.1 87.9 (87.2)+0.8 0.96 (1.19) +0.23 50/50 55 (55) 0 73.4 (72.2) +1.2 88.6 (87.9) +0.70.85 (1.12) +0.27 C95 33 72.0 86.2 1.52 38 73.1 87.0 1.40 42 74.0 87.71.32 Clay E 57 70.0 86.8 1.04 63 70.7 87.6 0.94 67 71.2 88.3 0.87 ClayE/ 50 (51) −1 73.0 (71.7) +1.3 87.5 (87.1) +0.4 1.12 (1.19) +0.07OptiCalGloss 57 (57) 0 74.1 (72.9) +1.2 88.5 (88.1) +0.4 1.00 (1.07)+0.07 50/50 63 (6.1) +2 74.9 (73.7) +1.2 89.3 (88.9) +0.4 0.92 (1.00)+0.08 Clay E/ 50 (50) 0 73.2 (72.4) +0.8 87.7 (87.4) +0.3 1.05 (1.16)+0.11 OptiCalPrint 56 (56) 0 74.3 (73.5) +0.8 88.9 (88.2) +0.7 0.97(1.05) +0.08 50/50 61 (60) +1 75.2 (74.3) +0.9 89.8 (89.0) +0.8 0.90(0.97) +0.07 Clay E/ 44 (45) −1 72.2 (71.0) +1.2 87.3 (86.5) +0.8 1.11(1.28) +0.17 C95 50 (50) 0 73.1 (71.9) +1.2 88.2 (87.3) +0.9 0.99 (1.17)+0.18 50/50 55 (55) 0 73.9 (72.6) +1.3 88.9 (88.0) +0.9 0.90 (1.09)+0.19 Clay C 53 70.8 86.7 1.07 60 71.8 87.8 0.95 64 72.5 88.5 0.87 ClayC/ 49 (49) 0 73.2 (72.0) +1.2 87.5 (87.0) +0.5 1.08 (1.21) +0.13OptiCalGloss 55 (55) 0 74.4 (73.5) +0.9 88.4 (88.2) +0.2 0.97 (1.07)+0.10 50/50 60 (60) 0 75.3 (74.4) +0.9 89.2 (89.0) +0.2 0.87 (0.97)+0.10 Clay C/ 51 (49) +2 73.6 (72.8) +0.8 87.9 (87.4) +0.5 1.03 (1.17)+0.14 OptiCalPrint 57 (55) +2 74.9 (74.0) +0.9 88.8 (88.4) +0.4 0.91(1.05) +0.14 50/50 61 (59) +3 75.9 (75.0) +0.9 89.6 (89.1) +0.5 0.82(0.97) +0.15 Clay C 41 (43) −2 72.5 (71.4) +1.1 87.0 (86.5) +0.5 1.20(1.29) +0.09 C95 47 (49) −2 73.4 (72.4) +1.0 87.8 (87.4) +0.4 1.09(1.17) +0.08 50/50 52 (53) −1 74.1 (73.2) +0.9 88.5 (88.1) +0.4 1.00(1.09) +0.09 Clay B 57 70.1 86.5 1.00 62 70.6 87.4 0.87 66 71.0 88.20.77 Clay B/ 57 (51) +6 73.2 (71.7) +1.5 87.8 (86.9) +0.9 1.04 (1.18)+0.14 OptiCalGloss 62 (56) +6 74.3 (72.9) +1.4 88.8 (88.0) +0.8 0.93(1.03) +0.10 50/50 66 (60) +6 75.0 (73.6) +1.4 89.7 (88.8) +0.9 0.84(0.93) +0.09 Clay B/ 48 (51) −3 72.9 (72.4) +0.6 87.7 (87.3) +0.4 1.08(1.14) +0.06 OptiCalPrint 55 (56) −1 74.2 (73.4) +0.8 88.7 (88.2) +0.50.93 (1.02) +0.09 50/50 59 (60) −1 75.3 (74.2) +1.1 89.4 (88.9) +0.50.84 (0.92) +0.08 Clay B/C95 46 (45) +1 72.3 (71.1) +1.2 87.0 (86.4)+0.6 1.13 (1.26) +0.13 50/50 51 (50) +1 73.1 (71.9) +1.2 87.9 (87.2)+0.7 1.05 (1.13) +0.08 55 (54) +1 73.8 (72.5) +1.3 88.7 (88.0) +0.7 0.97(1.04) +0.07

Example 3

In this Example, the properties of compositions in which the particulatepigment comprises an aragonitic precipitated calcium carbonate and akaolin clay having a shape factor of 25 and a steepness above 20 (ClayF), were measured at different clay:PCC ratios in comparison to acomposition including rhombohedral precipitated calcium carbonate inplace of aragonitic PCC at the 60:40 clay:PCC ratio and compositionsincluding single-component pigments.

For the blends, two numbers are listed for each property. These arefirstly the measured property, then (in brackets) the arithmetic meancalculated from the results for the 100% components.

The composition and coating conditions were as stated in the heading toTable 6 below, which shows the results obtained. TABLE 6 LWC - 8 gsmFormulation: 8 parts Dow 950 latex, 9 parts PG 280 Starch + StearateHelicoating: 600 m/min, 45° Blade Angle, Caledonian LWX BaseSupercalendering: 8 nips, 1000 psi, 65° C. Calendered Sheet ColourProperties Solids B/ness Opacity Print Gloss Print Density Pigment BlendWt % Gloss % (ISO) (ISO) Dry Litho Dry L/D Clay F 55.4 61 71.8 91.4 8776 1.59 0.72 Carbonate D 56.0 54 77.0 92.3 70 67 1.45 0.97 Carbonate E56.0 44 77.2 92.3 71 69 1.42 0.96 Clay F/Carb D 56.0 56 74.6 92.0 77 671.51 0.88 60:40 (58) (73.9) (91.8) (88) (72) (1.53) Clay F/Carb D 56.457 75.7 92.1 75 67 1.50 0.87 40:60 (57) (74.9) (91.9) (77) (71) (1.51)Clay F/Carb E 56.2 50 75.0 91.8 72 65 1.47 0.90 60:40 (54) (74.0) (91.8)(81) (73) (1.52)

Example 4

In this example, the properties of compositions according to theinvention, in which the particulate pigment comprises an aragonitic orrhombohedral precipitated calcium carbonate and a kaolin clay having ashape factor of 25 to 30 and steepness of greater than 20 (clay H) weremeasured at 50:50 blend ratios in comparison to compositions includingsingle-component pigments.

For the blends, two numbers are listed for each property. These arefirstly the measured property, then (in brackets) the arithmetic meancalculated from the results for the 100% components.

The composition and coating conditions were as stated in the heading toTable 7 below, which shows the results obtained. Table 8 belowsummarises the observed synergies. TABLE 7 LWC Formulation: 10 parts Dow950 latex, 0.3 parts Finnfix 5 CMC Coating: 600 m/min, 45° Blade Angle,LWC Base Soft Calendering: 20 m/min 1 nip, 300 kN/m², 100° C. SheetProperties Print Properties Colour Sheet Sheet Whiteness Coat SolidsSheet B'ness Opacity (−UV) Weight Print Gloss Print Density Colour Wt %Gloss % ISO ISO (D65) GSM Dry Litho Snap Dry Litho L/D Sole PigmentsCarbonate H 66.5 55 76.3 86.0 64.3 8.5 68 63 13 1.42 1.41 0.99 CarbonateG 64.3 53 76.4 86.0 65.0 8.3 67 62 15 1.45 1.42 0.98 Carbonate F 64.3 4876.7 85.5 65.0 8.3 65 59 17 1.42 1.40 0.99 Carbonate I 66.8 50 76.5 85.363.7 7.7 68 63 18 1.44 1.40 0.97 Carbonate E 67.1 53 77.1 86.4 64.3 8.171 68 18 1.43 1.40 0.98 Carbonate C 69.3 53 75.2 84.7 61.3 8.5 69 65 161.50 1.44 0.96 Clay H 61.7 61 73.0 86.2 57.4 7.7 80 64 19 1.50 1.21 0.8150:50 Blends with Clay H Carbonate H 63.8 63 75.5 87.0 62.8 7.4 78 65 151.46 1.37 0.94 (58) (74.7) (86.1) (60.9) (74) (1.46) Carbonate G 62.8 6075.4 86.5 62.4 8.2 77 67 17 1.47 1.38 0.94 (57) (74.7) (86.1) (61.2)(74) (1.48) Carbonate F 62.6 57 75.7 86.7 62.6 7.7 76 64 19 1.45 1.360.94   (54.5) (74.9) (85.9) (61.2) (72) (1.46) Carbonate I 63.7 55 75.686.7 62.0 8.2 77 69 22 1.48 1.36 0.92   (55.5) (74.8) (85.8) (60.6) (74)(1.47) Carbonate E 64.1 55 76.0 87.2 62.4 7.9 77 66 22 1.45 1.34 0.92(57) (75.1) (86.3) (60.9) (76) (1.47) Carbonate C 65.2 55 75.2 86.3 61.17.7 75 64 20 1.47 1.36 0.93 (57) (74.1) (85.0) (59.4) (75) (1.50)

TABLE 8 Sheet Sheet Whiteness Sheet B'ness Opacity (−UV) Print ColourGloss % ISO ISO (D65) Gloss Carbonate H +5 +0.8 +0.9 +1.9 +4 Carbonate G+3 +0.7 +0.4 +1.2 +3 Carbonate F +3 +0.8 +0.8 +1.4 +4 Carbonate I 0 +0.8+0.9 +1.4 +3 Carbonate E −2 +0.9 +0.9 +1.5 +1 Carbonate C −2 +1.1 +1.3+1.7 0

Example 5

In this Example, the effect on the synergies of varying the proportionsof first:second components of the pigment blend was investigate inrelation to Clay G and aragonitic PCC.

The composition and coating conditions were as stated in the heading toTable 9 below, which shows the results obtained. Table 10 belowsummarises the observed synergies. TABLE 9 LWC Formulation: 11 parts Dow950 latex, 0.3 parts Finnfix 5 CMC Coating: 600 m/min, 45o Blade Angle,LWC Base Supercalendering: Standard Offset Conditions Colour Sheet SheetSolids Sheer Brightness Opacity Colour Wt % Gloss % ISO ISO Carbonate J64.9 56 76.3 86.8 Clay G 61.3 68 73.0 86.7 50:50 Carb. J:Clay G 62.6 6675.5 87.4 (62) (74.7) (86.8) 75:25 Carb. J:Clay G 63.6 61 76.0 87.0 (59)(75.5) (86.8)

TABLE 10 Sheet Brightness Sheet Opacity Colour Sheet Gloss % ISO ISO50:50 Blend +4 +0.8 +0.6 75:25 Blend +2 +0.5 +0.2

Example 6

In this example, the effect on the synergies of varying proportions offirst:second components of the pigment blend was investigated inrelation to Clay H and aragonitic PCC. Table 11 below summarises theobserved synergies. TABLE 11 LWC Formulation: 10 parts Dow 950 latex, 4parts Cerestar (05598) starch + cross-linker Coating: LDTA, 1400 m/min,48° Blade Angle, 0.381 mm Blade, LWC Base (39 gsm) Supercalendering: 800m/min 11 nips, 300 kN/m², 100° C. PCC/ Colour Sheet Sheet Sheet SheetClay Solids Gloss Gloss B'ness Opacity Print Gloss Print Density HRatioWt % Side 1 Side 2 ISO ISO Dry Litho Dry L/D 30:70 56.6 62 54 71.6 91.871 59 1.43 0.87* 50:50 59.7 64 59 72.4 92.1 73 63 1.47 0.92 70:30 60.058 50 72.9 91.5 67 60 1.45 0.96

It is seen that optimisation occurs around the 50:50 blend ratio. Thisratio was selected for the following example.

Example 7

In this Example, the selected 50:50 ratio of calcium carbonate wascompared against the comparison formulations:

Carbonate F:Clay K

Carbonate I:Clay K

Carbonate K:Clay K

The composition and coating conditions were as stated in the heading toTable 12 below. TABLE 12 LWC Formulation: 10 parts Dow 950 latex, 4parts Cerestar (05598) starch + cross-linker Coating: LDTA, 1400 m/min,48° Blade Angle, 0.381 mm Blade, LWC Base (39 gsm) Supercalendering: 800m/min 11 nips, 300 kN/m², 100° C. Colour Sheet Sheet Sheet Sheet PrintPrint Colour (all Solids Gloss Gloss B'ness Opacity Gloss Density 50:50blends) Wt % Side 1 Side 2 ISO ISO Dry Litho Dry L/D Carb F/Clay H 59.764 59 72.4 92.1 73 63 1.47 0.92 Carb F/Clay K 60.0 67 60 71.7 91.7 66 601.42 0.96 Carb. I/Clay K 61.2 66 59 71.1 91.3 69 63 1.42 0.95 Carb.K/Clay K 61.3 57 50 70.4 90.9 61 56 1.42 0.96

Example 8

In this Example, 80:20 blends of calcium carbonate:clay using CarbonateM and Clays G and H were compared against the comparison formulations:

Carbonate M:Clay K

Carbonate K:Clay K

Carbonate L:Clay K

Carbonate I:Clay K

Two different topcoating techniques were used, the results being shownin Tables 13 and 14 below and the compositions and coating conditionswere as stated in the respective leadings to those tables. TABLE 13Topcoating Formulation: 10 parts Dow 950 latex, 0.3 parts Finnfix 5 CMCand 0.5 parts OBA Coating: LDTA, 800 m/min, Bent Blade (0.381 mm),Precoated Woodfree Base Supercalendering: 800 m/min 11 nips, 300 kN/m²,100° C. Colour Colour Sheet Sheet W'ness Print Print All 80 parts SolidsSheet B'ness Opacity (+UV) Gloss Density Carbonate Wt % Gloss % ISO ISOD65 Dry Litho Dry L/D Carb. M/Clay H 60.3 80 87.4 89.2 107.6 93 88 1.540.97 Carb. M/Clay G 61.8 79 87.5 89.5 107.4 93 89 1.53 0.97 Carb. M/ClayK 62.9 79 86.9 89.7 104.8 90 87 1.54 0.98 Carb. K/Clay K 67.3 77 86.988.4 106.9 94 89 1.56 0.97 Carb. L/Clay K 66.6 78 87.3 88.7 108.7 93 911.55 0.97 Carb. I/Clay K 64.0 76 87.3 89.3 103.6 93 88 1.51 0.96

TABLE 14 Topcoating Formulation: 10 parts Dow 950 latex, 0.3 partsFinnfix 5 CMC and 0.5 parts OBA Coating: 600 m/min, 45° angle, 65 gsmUncoated Woodfree Base Soft Calendering: 800 m/min, 1 nips, 300 kN/m²,100° C. Colour Roughness Sheet Sheet W'ness Solids Sheet PPS B'nessOpacity (+UV) Print Gloss Colour Wt % Gloss % 5 kPa ISO ISO (D65) DryLitho Carb. K/Clay K 68.3 76 0.68 81.4 86.4 92.9 91 87 70:30 Carb.M/Clay G 62.2 77 0.60 82.4 87.8 92.5 91 88 50:50 Carb. M.Clay G 65.1 740.73 83.2 87.4 95.7 90 87 70:30

Example 9

This example illustrates the performance of a 50:50 mixture of aragoniteand rhombohedral PCC in a pigment containing a blocky particulate kaolin(Clay K).

A 75 gsm pre-coated woodfree base was coated on a Heli-Coater™ using ablade applicator at 1200 m/min with the coatings being run at themaximum runnable solids. The formulation was 83 parts carbonate and 17parts kaolin with 9 parts of latex (4.5 pph styrene acrylic latexAcronal S360D; 4.5 pph styrene butadiene latex Dow DL940), 1 part PVOH,0.6 parts OBA (Tinopal ABP), 0.3 parts CMC and 0.6 parts calciumstearate at ph 8.5. The coat weight range was 8-12 gsm and the data wereinterpolated to 10 gsm.

The kaolins were Clays J and K. The PCC was prepared from Carbonates Aand B. The results are shown in Table 15 below. TABLE 15 Bright- Glossness Opacity Print gloss Snap (print Pigment (TAPPI) (ISO) (TAPPI)(TAPPI) gloss-gloss) Conventional 79 91.8 88.3 88 9 Fine ground GCC/ClayK Control Rhombo/ 78 92.1 89.4 89 11 Aragonite 50:50/ Clay K

As shown in Table 16 below, similar behaviour was observed when thekaolin was changed to Clay J. TABLE 16 Snap Print (print GlossBrightness Opacity gloss gloss- Pigment (TAPPI) (ISO) (TAPPI) (TAPPI)print) Rhombo/ 78 92.1 89.4 89 11 Aragonite 50:50/Clay JThe above data illustrate the desirable effect of using a 1:1 wt./wt.blend of aragonite and rhombohedral PCC. This gave, compared to the GCCcontrol; minus one unit of gloss, +0.3 units of brightness, +1.1 unitsof opacity, +1 unit of print gloss and a snap value of +11 compared to+9 with the control.DiscussionThis work confirms that synergistic advantages in the properties ofgloss, brightness, opacity and smoothness, or at least some of them,occur when the combinations of particulate calcium carbonate andparticulate kaolin clays according to the present invention are employedas pigments in paper coating compositions.Generally speaking, the advantages are shown at all conventional coatingweights on the paper. However, when the particulate kaolin clay has arelatively high shape factor, simultaneously with a relatively low meanequivalent particle diameter and relatively high steepness, theadvantages are more pronounced at higher coating weights.Generally speaking, aragonitic precipitated calcium carbonate ispreferred as the first component of the pigment system according to thepresent invention. The ratio of calcium carbonate to kaolin clay issuitably around 50:50.

1-14. (canceled)
 15. A pigment composition comprising: (a) aprecipitated calcium carbonate comprising particle shapes chosen frompredominantly aragonitic, predominantly rhombohedral, and mixturesthereof, and (b) a kaolin clay with a shape factor greater than or equalto about 25 and a steepness greater than or equal to about
 20. 16. Thecomposition of claim 15, wherein the precipitated calcium carbonatecomprises a predominantly rhombohedral precipitated calcium carbonate.17. The composition of claim 16, wherein the predominantly rhombohedralprecipitated calcium carbonate has a d₅₀ of less than about 0.8 microns.18. The composition of claim 16, wherein the predominantly rhombohedralprecipitated calcium carbonate has a d₅₀ of less than about 0.7 microns.19. The composition of claim 16, wherein the predominantly rhombohedralprecipitated calcium carbonate has a d₅₀ of at least about 0.2 microns.20. The composition of claim 16, wherein the predominantly rhombohedralprecipitated calcium carbonate has a d₅₀ ranging from about 0.25 micronsto about 0.45 microns.
 21. The composition of claim 16, wherein thepredominantly rhombohedral precipitated calcium carbonate has a d₅₀ranging from about 0.4 microns to about 0.6 microns.
 22. The compositionof claim 16, wherein the predominantly rhombohedral precipitated calciumcarbonate has a particle size distribution such that at least about 93%by weight of the particles have an equivalent spherical diameter lessthan 2 microns.
 23. The composition of claim 16, wherein thepredominantly rhombohedral precipitated calcium carbonate has a particlesize distribution such that at least about 86% by weight of theparticles have an equivalent spherical diameter less than 1 micron. 24.The composition of claim 16, wherein the predominantly rhombohedralprecipitated calcium carbonate has a particle size distribution suchthat at least about 22% by weight of the particles have an equivalentspherical diameter less than 0.5 microns.
 25. The composition of claim16, wherein the predominantly rhombohedral precipitated calciumcarbonate has a particle size distribution ranging from 5% to 25% byweight of the particles have an equivalent spherical diameter less than0.25 microns.
 26. The composition of claim 16, wherein the predominantlyrhombohedral precipitated calcium carbonate has a GE brightness of atleast
 90. 27. The composition of claim 16, wherein the predominantlyrhombohedral precipitated calcium carbonate has a GE brightness of atleast
 92. 28. The composition of claim 16, wherein the kaolin clay has ashape factor greater than about
 25. 29. The composition of claim 16,wherein the predominantly rhombohedral precipitated calcium carbonatehas a particle size distribution such that: at least 93% by weight ofthe particles have an equivalent spherical diameter of less than 2microns; at least 86% by weight of the particles have an equivalentspherical diameter of less than 1 micron; at least 22% by weight of theparticles have an equivalent spherical diameter of less than 0.5microns; and from 5% to 25% by weight of the particles have anequivalent spherical diameter less than 0.25 microns.
 30. Thecomposition of claim 15, wherein the precipitated calcium carbonatecomprises a predominantly aragonitic precipitated calcium carbonate. 31.The composition of claim 30, wherein the predominantly aragoniticprecipitated calcium carbonate has a d₅₀ of less than about 0.8 microns.32. The composition of claim 30, wherein the predominantly aragoniticprecipitated calcium carbonate has a d₅₀ of less than about 0.7 microns.33. The composition of claim 30, wherein the predominantly aragoniticprecipitated calcium carbonate has a d₅₀ of at least about 0.2 microns.34. The composition of claim 30, wherein the predominantly aragoniticprecipitated calcium carbonate has a d₅₀ ranging from 0.25 microns toabout 0.45 microns.
 35. The composition of claim 30, wherein thepredominantly aragonitic precipitated calcium carbonate has a particlesize distribution such that at least about 90% by weight of theparticles have an equivalent spherical diameter less than 2 microns. 36.The composition of claim 30, wherein the predominantly aragoniticprecipitated calcium carbonate has a particle size distribution suchthat at least about 75% by weight of the particles have an equivalentspherical diameter less than 1 micron.
 37. The composition of claim 30,wherein the predominantly aragonitic precipitated calcium carbonate hasa particle size distribution such that at least about 60% by weight ofthe particles have an equivalent spherical diameter less than 0.5microns.
 38. The composition of claim 30, wherein the predominantlyaragonitic precipitated calcium carbonate has a particle sizedistribution ranging from 15% to 40% by weight of the particles have anequivalent spherical diameter less than 0.25 microns.
 39. Thecomposition of claim 30, wherein the predominantly aragoniticprecipitated calcium carbonate has a GE brightness of at least
 90. 40.The composition of claim 30, wherein the predominantly aragoniticprecipitated calcium carbonate has a GE brightness of at least
 92. 41.The composition of claim 30, wherein the kaolin clay has a shape factorgreater than about
 25. 42. The composition of claim 30, wherein thepredominantly aragonitic precipitated calcium carbonate has a particlesize distribution such that: at least 90% by weight of the particleshave an equivalent spherical diameter of less than 2 microns; at least75% by weight of the particles have an equivalent spherical diameter ofless than 1 micron; at least 60% by weight of the particles have anequivalent spherical diameter of less than 0.5 microns; and from 15% to40% by weight of the particles have an equivalent spherical diameterless than 0.25 microns.
 43. The composition of claim 30, wherein thepredominantly aragonitic precipitated calcium carbonate has a particlesize distribution such that: at least 95% by weight of the particleshave an equivalent spherical diameter of less than 2 microns; at least82% by weight of the particles have an equivalent spherical diameter ofless than 1 micron; at least 66% by weight of the particles have anequivalent spherical diameter of less than 0.5 microns; and from 23% to33% by weight of the particles have an equivalent spherical diameterless than 0.25 microns.
 44. The composition of claim 15, wherein thekaolin clay has a shape factor greater than about
 30. 45. Thecomposition of claim 15, wherein the kaolin clay has a shape factorgreater than about
 35. 46. The composition of claim 15, wherein thekaolin clay has a shape factor greater than about
 45. 47. Thecomposition of claim 15, wherein the kaolin clay has a d₅₀ of less thanabout 0.5 microns.
 48. The composition of claim 15, wherein the kaolinclay has a d₅₀ ranging from about 0.1 microns to about 0.5 microns. 49.The composition of claim 15, wherein the kaolin clay has a d₅₀ ofgreater than about 0.5 microns.
 50. The composition of claim 15, whereinthe kaolin clay has a d₅₀ ranging from about 0.5 microns to about 1.5microns.
 51. The composition of claim 15, wherein the kaolin clay has asteepness ranging from about 25 to about
 45. 52. The composition ofclaim 15, wherein the kaolin clay has a steepness ranging from about 35to about
 45. 53. The composition of claim 15, wherein the kaolin claycomprises at least 50% by weight kaolinite.
 54. The composition of claim15, wherein the kaolin clay comprises greater than 75% by weightkaolinite.
 55. The composition of claim 15, wherein the kaolin claycomprises greater than 90% by weight kaolinite.
 56. The composition ofclaim 15, wherein the kaolin clay has a GE brightness of at least 85.57. The composition of claim 15, wherein the kaolin clay has a GEbrightness of at least
 90. 58. The composition of claim 15, wherein theprecipitated calcium carbonate comprises at least about 40% by weightrelative to the total composition.
 59. The composition of claim 15,wherein the precipitated calcium carbonate comprises at least about 70%by weight relative to the total composition.
 60. The composition ofclaim 15, wherein the precipitated calcium carbonate comprises not morethan about 75% by weight relative to the composition.
 61. A coatingcomposition for paper and other substrates, the composition comprisingan aqueous suspension of a particulate pigment and a binder, wherein theparticulate pigment comprises: (a) a precipitated calcium carbonatecomprising particle shapes chosen from predominantly aragonitic,predominantly rhombohedral, and mixtures thereof, and (b) a kaolin claywith a shape factor greater than or equal to about 25 and a steepnessgreater than or equal to about
 20. 62. The composition according toclaim 61, wherein the binder comprises a modified starch.
 63. Thecomposition according to claim 61, further comprising at least oneadditional component chosen from: cross linkers; water retention aids;viscosity modifiers and thickeners; lubricity/calendering aids;dispersants; antifoamers/defoamers; dry and wet pick improvementadditives; dry and wet rub improvement and/or abrasion resistanceadditives; gloss-ink hold-out additives; optical brightening agents(OBA) and/or fluorescent whitening agents (FWA); dyes; biocides/spoilagecontrol agents; levelling and evening aids; grease and oil resistanceadditives; water resistance additives; additional pigments; and mixturesthereof.
 64. The composition according to claim 63, consistingessentially of the aqueous suspension of the particulate pigment, thebinder, and the at least one additional component, with less than about10% by weight of the at least one additional component.
 65. A method forpreparing a coating composition comprising an aqueous suspension of aparticulate pigment and a binder, wherein the particulate pigmentcomprises: a precipitated calcium carbonate comprising particle shapeschosen from predominantly aragonitic, predominantly rhombohedral, andmixtures thereof, and a kaolin clay with a shape factor greater than orequal to about 25 and a steepness greater than or equal to about 20,comprising: mixing the particulate pigment and the binder into anaqueous liquid medium to prepare a suspension of the solid componentstherein.
 66. A method for preparing a coated gloss paper comprising:applying to the paper a composition comprising an aqueous suspension ofa particulate pigment and a binder, wherein the particulate pigmentcomprises: a precipitated calcium carbonate comprising particle shapeschosen from predominantly aragonitic, predominantly rhombohedral, andmixtures thereof, and a kaolin clay with a shape factor greater than orequal to about 25 and a steepness greater than or equal to about 20 tocoat the paper, and calendering the paper to form a gloss coatingthereon.
 67. A paper coated with a gloss coating comprising a dryresidue of a composition comprising an aqueous suspension of aparticulate pigment and a binder, wherein the particulate pigmentcomprises: a precipitated calcium carbonate comprising particle shapeschosen from predominantly aragonitic, predominantly rhombohedral, andmixtures thereof, and a kaolin clay with a shape factor greater than orequal to about 25 and a steepness greater than or equal to about
 20. 68.The paper according to claim 67, which is a coated mechanical paper. 69.The paper according to claim 67, which is a coated lightweight coatedpaper (LWC).